As it is known in the art, patients suffering from kidney failure or renal insufficiency, or patients suffering of particular pathologies should be submitted to specific treatments. More in detail, it is known to treat blood in an extracorporeal circuit in order to carry out ultrafiltration, haemodialysis, haemofiltration, haemodiafiltration, plasmapheresis, blood component separation, blood oxygenation, etc.
Extracorporeal blood treatment means taking the blood from a patient, treating the blood outside the patient, and returning the treated blood to the patient. Normally, blood is removed from a blood vessel, sent into a withdrawal line of an extracorporeal circuit, passed through a blood-treating unit and returned to another or to the same blood vessel.
Extracorporeal blood treatment is typically used to extract undesirable matter or molecules from the patient's blood, and/or to add beneficial matter or molecules, to the blood. Extracorporeal blood treatment is used with patients incapable of effectively eliminating matter from their blood, for example in the case of a patient who is suffering from temporary or permanent kidney failure. These and other patients may undergo extracorporeal blood treatment to add to or to eliminate matter from their blood, to maintain an acid-base balance or to eliminate excess body fluids, for instance. Extracorporeal blood treatment is typically performed by sampling the patient's blood in a continuous flow, by introducing the blood into a primary chamber of a blood-treating unit in which the blood goes through a semi-permeable membrane. The semi-permeable membrane selectively lets the unwanted matter contained in the blood pass through the membrane, from the primary chamber to the secondary chamber, and also selectively lets the beneficial matter contained in the liquid going into the secondary chamber pass through the membrane to the blood going into the primary chamber, according to the type of treatment.
A number of extracorporeal blood treatments may be performed by the same machine. In ultrafiltration (UF) treatment, the unwanted matter is eliminated from the blood by convection through the membrane in the secondary chamber.
In haemofiltration (HF) treatment, the blood runs through the semipermeable membrane as in UF, and the beneficial matter is added to the blood, typically by the introduction of a fluid into the blood, either before, or after its passage through the blood-treating unit and before it is returned to the patient. In haemodialysis (HD) treatment, a secondary fluid containing the beneficial matter is introduced into the filter's secondary chamber. The blood's unwanted matter crosses the semi-permeable membrane and penetrates into the secondary fluid, and the beneficial matter of the secondary fluid may cross the membrane and penetrate into the blood.
In haemodiafiltration (HDF) treatment, the blood and the secondary fluid exchange their matter as in HD, and further, matter is added to the blood, typically by introducing a fluid into the treated blood before it is returned to the patient as in HF, and the unwanted matter is also eliminated from the blood by convection.
In each treatment, the secondary fluid goes through the filter's secondary chamber and receives the blood's unwanted matter by means of the membrane. This liquid is then extracted from the filter: it is commonly called waste, and is sent to a drain or to a receptacle then intended to be discharged into a drain.
In fluid transport lines of medical devices, pumps are usually used in order to pump different fluids, such as blood, treatment liquids, waste liquids, along the transport lines. Such pumps may be for example peristaltic pumps, volumetric pumps, piston type pumps, etc. . . . .
Document U.S. Pat. No. 4,747,950 discloses a method and apparatus for controlling ultra filtration during haemodialysis. Such apparatus comprises a receptacle in fluid communication with a reservoir and a first metering pump provided to withdraw fresh dialysate from the reservoir and deliver it to the receptacle, at a predetermined fixed rate. A second pump is provided to withdraw fresh dialysate from the receptacle and to deliver it to a haemodialyzer. A liquid sensing device is included in the receptacle and the second pump is for example a variable flow pump controllable to adjust the rate of flow of the fresh dialysate towards the haemodialyzer in order to maintain the level or volume of fresh dialysate in the receptacle at a predetermined constant level.
Document U.S. Pat. No. 4,372,846 discloses a blood purification system comprising filtrate metering means including a small container with an upper liquid level sensor and a lower liquid level sensor, the container having an inlet channel and an outlet channel provided with a valve. When the container is filled so that the fluid reaches a position above the level where the upper liquid level sensor is located, the valve is opened to discharge the content of the container. When the liquid subsequently reaches a position below the level where the lower liquid level sensor is located, the valve is closed again.
Document U.S. Pat. No. 6,440,311 discloses a system and method for monitoring a dosage pump in a dialysis machine, in which a second pump is arranged between a dosage pump and a source of fluid and a slave chamber is arranged between the dosage pump and the second pump. A level detector is arranged in the slave chamber to detect when the level of fluid in the slave chamber goes below its level due to a stroke of the piston type dosage pump, and a control arrangement is arranged for activating the second pump when the level is below the level detector. The relationship between the flow from the slave chamber to the dosage pump during its suction stroke and the flow to the slave chamber from the second pump is such that the level in the slave chamber never reaches above the level detector.
Document WO 91/15253, having the features according to the preamble of claim 1, relates to a system for weighing and monitoring a flow from multiple fluid sources into a flow system wherein incoming fluid is received in a weighing bag attached to a control system for monitoring the amount of fluid passed through the same weighing bag. According to what described replacement fluid is sent from replacement fluid containers to a weighing bag by opening a respective clamp. A weight scale measures the fluid contained in the weighing bag after the clamp is closed. A replacement fluid pump is then activated and the weight variation in the weighing bag is used to continuously calibrating the replacement fluid pump to compensate for changes in the resiliency of the tubing set caused, for example, by temperature change.
The known devices suffer generally of one or more of the following problems.
It is to be noted that in known extracorporeal blood treatments the determination of the exact fluid flow rate of the various fluids in the transport lines is a critical aspect, since the amount of liquids passing in the lines is strictly correlated to the quality of the medical treatment. In fact, any imprecision in the fluid rate passing in any of the circuit lines can impair the quality of the medical treatment and may also lead to serious consequences for the health of the patient.
Consequently in known medical devices only pumps having a high level of accuracy and reliability shall be used, and in any case it is difficult to check the accuracy of the actual fluid flow rate passing through the pump.
Indeed the control means of the medical machine usually monitors the pumps and/or the flow rates through sensors.
For example the peristaltic pump speed is monitored (with Hall sensors or the like) and is related to the fluid flow rate in the tube that the pumps is acting on.
In any case the control means cannot check the accuracy of said measure and can not determine any mistake in said measure.
Particularly accuracy is affected by several technical features such as correct coupling between tube and pump, other device elements placed along the tube/line, modification of the working parameters, etc. . . . .